When a semiconductor wafer is processed into integrated circuits (ICs), it is necessary to thoroughly wash the wafer in order to remove residual chemicals, small particles and other contaminants which occur during various steps of processing. It is important that the face of the wafer be washed ultra-clean so that even microscopic traces of contaminants are completely removed.
There are a number of semiconductor wafer washing and drying systems commercially available. The present invention is applicable to those systems which utilize de-ionized water (DIW) in combination with an organic vapor such as isopropyl alcohol (IPA) in an atmosphere such as dry nitrogen. Such a system washes and dries a wafer or wafers in accordance with the "Marangoni effect" which will be further described hereinafter.
As is well known, when a semiconductor wafer is processed into ICs, a face of the wafer is contoured into microscopically fine details with trenches, and plateaus lines, holes, etc. As the ICs become denser and denser, these surface details become smaller. For example in the case of a high-density DRAM (dynamic random access memory), the trenches in the silicon face of the wafer between capacitor stacks may be finer than one-half micron. Water washing liquid, such as de-ionized water (DIW) collects in these trenches and on other surface details and is hard to remove completely because of capillary action of the wafer-washing liquid.
De-ionized water (DIW) is an aggressive solvent and will even dissolve small amounts of the silicon of a wafer. Thus, if after washing of the wafer, even small amounts of DIW are trapped in trenches or on other surface details of ICs on the wafer, small amounts of silicon are dissolved into the DIW. When this trapped DIW is subsequently dried by evaporation, silicon compounds are left behind as deposits commonly called watermarks (mostly from DIW but partly from DIW mixed with IPA on the surface of the wafer). These watermarks, in turn, lead to defects in the ICs. It is highly desirable therefore that the washing and drying of a semiconductor wafer be such that even small amounts of DIW be removed from the surface details of the ICs before watermarks can be formed. A further discussion of watermarks is to be found in an article entitled "Effects of Drying Methods and Wettability of Silicon on the Formation of Water Marks in Semiconductor Processing," by Jin-Goo Park and Michael F. Pas, pages 2028-2031, Journal Electrochemical Society, Vol. 142, No. 6, June 1995.
When a semiconductor wafer, held vertical, is slowly drawn out of a tank of wash water (e.g., DIW), there is a meniscus of water at the face of the wafer. The meniscus forms a thin zone of water vertically against the face of the wafer; this zone rises slightly above the top surface level of the main body of water in the tank. This is a well known phenomenon. If the surface of the water is also exposed to an organic vapor, such as IPA in dry nitrogen, molecules of the vapor continually diffuse into the water. These molecules are concentrated to a much greater extent at or near the surface of the water than deeper into it. Thus the thin zone of the water meniscus against the face of the wafer has a very much higher concentration of organic molecules (IPA) than does the main body of water (DIW) in the tank.
Isopropyl alcohol (IPA) has a lower surface tension (ST) than does de-ionized water (DIW). For example, at 20.degree. C. the ST of IPA is about 21 dynes/cm, whereas the ST of DIW is about 72 dynes/cm. Because of this fact there is a differential between the lowered surface tension (ST) of the liquid (DIW plus high concentration of IPA) in the zone of the meniscus at the surface of the wafer and the higher surface tension (ST) of the main body of wash water (DIW alone). As a result of this differential of STs, there is a flow of liquid internally from the zone of the meniscus into the main body of water. This is called the "Marangoni effect" and is well known in the art (see for example, U.S. Pat. No. 5,569,330, dated Oct. 29, 1996, and U.S. Pat. No. 5,571,337, dated Nov. 5, 1996). By using the "Marangoni effect", water is more efficiently removed from the face of a wafer than otherwise. Wafer washing and drying apparatus utilizing the "Marangoni effect" is commercially available.
The present invention provides improved apparatus and method for washing and drying semiconductor wafers utilizing liquid flow offof the wafer by means of an enhanced "Marangoni effect". This, in combination with other features of the invention, provides substantially better prevention of watermarks than do previous systems.